The purpose of this study is to elucidate the mechanisms by which immunoglobulins interact with effector cells of the immune system subsequent to complexing with antigens. A simple model describes the interaction of IgG oligomers with mobile cell-surface Fc receptors. The model correctly predicts that the release of bound oligomer from cell surfaces subsequent to washing should be slow at 0 percent. This feature of oligomer binding has permitted the development of flow microfluorometric methods for measuring the distribution of Fc receptors within homogeneous and heterogeneous cell population. Certain procedures can totally prevent the release of bound immunoglobulins from cell surfaces. When antibody is fixed to the surfaces of receptor-bearing cells by such procedures, specific killer cells are generated. These killer cells, unlike normal ADCC effector cells, cannot be inhibited from lysing given targets by immune complexes.